In a marine vessel, it is not unusual to draw water into the vessel through a through-hull fitting for use, for example, as cooling water for an engine or for other purposes. The water is normally conducted through a strainer and then is piped to the engine or other device.
Whenever water is drawn in in this fashion, it is important to be sure that the water flow is adequate for the consumer, i.e., for the article using the water. If the consumer is the engine, it is essential for the flow to be adequate for the engine's needs to avoid possible severe damage to the engine. The same is true for other devices such as an air conditioning pump, generator, deckwash pump, desalination pump or the like. While the engine is obviously the most critical consumer, and generally uses more water than other devices, providing inadequate flow to any of the consumers can result in damage requiring expensive repairs.
Because of the critically of this flow, the use of a manifold is avoided. While a manifold could theoretically be used to provide water to an engine and also to two or more other devices, it is has been considered dangerous to do so because, if a manifold is used, it is possible for an installer of the equipment to select improper sizes of hoses or pipes to the engine and other consumers, allowing too little flow to the engine or one or more of the other devices. Thus, water is often drawn in through more than one through-hull fitting, one for each water consumer. While this works, it causes additional expense and additional openings through the hull, both of is which are undesirable.